5.3.2 Blade tip restraint
The use of long blades and large blade chords involves an increase in pitch and this creates complications in the various devices used to stiffen the blade assembly and dampen vibration. A coverband or lacing wire must behave as a beam spanning the blade pitch in resisting centrifugal loading, and must also accommodate the substantial circumferential strains due to radial elastic extension of the blades and the tendency of the blade to untwist at speed.
When lacing wires are used, they are usually of the 'loose' type with circumferential restraint on only one blade in each group, and are free to move circumferentially in adjacent blades, centrifugal forces providing the necessary damping through friction. The gaps between adjacent lacing wires can introduce complications in vibration patterns on long blades and overlapping lacing wires are sometimes used to give some measure of continuity around the annulus. A coverband of conventional design is not feasible for slim sections and where the peripheral speed may be approaching Mach 2, but a continuous ring of stiffening devices of sufficient elasticity may be used to accommodate circumferential strains. The elastic arch banding, shown in Fig 1.86, braces the blade tip and provides some resistance to blade untwist as well as largely permitting circumferential strain.
Zigzag spool rods shown in Fig 1.87 are frequently incorporated in the tips of the last-stage LP blades of modern turbines. The spool rods provide no restraint against circumferential expansion or centrifugal untwist, but the reduced sections at the ends of the rods are forced against the holes in the blades by centrifugal action and the sliding friction provides effective damping, minimising blade vibration and high frequency flutter at the blade tip.
A recent variant to the spool rod is the ball-jointed tip tie. The dumb-bell-shaped tie rod fits into a fixed ball-shaped socket in one blade and into a sliding ball socket in the next blade. In common with the spool rods, centrifugal strains are not restrained but the ball-joints provide more controlled damping and the evaluation of tie rod stressing is more reliable. Some thickening of the blade is required, however, in the region of the tip tie to accommodate the ball-joint sockets.
A few manufacturers have dispensed with all forms of stiffening, relying on the stiffening effect of centrifugal forces, which can be very substantial on freely-cantilevered blades. This type of blading tends to be wider and heavier than blading with stiffening and introduces difficulties in manufacture and rotor design. Freely-cantilevered blades are most suitable for the last stages of very small high speed turbines, where it is not possible to introduce complicated stiffening devices and where the increased blade width is not an embarrassment.